The present invention relates to a system for lubricating and cooling a mechanical assembly. Although not exclusively, it is particularly well suited to lubricating and cooling a transmission gearbox, particularly a main transmission gearbox, usually used on board helicopters. Thus, the present invention will be described more particularly hereinafter in relation to this application.
It is known that, for correct operation, such a transmission gearbox is lubricated and cooled, generally by a device which circulates a lubricating and cooling liquid, such as oil. In addition, to prevent said transmission gearbox from being able to be damaged or even destroyed through lack of lubrication and cooling in the event of this device failing during flight, a backup device is provided whose purpose is to, at least partially, lubricate and cool said transmission gearbox to allow the helicopter to continue to fly for a predetermined survival time.
For example, American patent 4,717,000 has already disclosed a system for lubricating and cooling helicopter transmission gearboxes comprising a main device which circulates a stream of oil under pressure through said transmission gearboxes and a backup device which is set in operation automatically or manually when said main device becomes defective. This backup device comprises:
a reservoir connected to said main device so as to fill with lubricating and cooling liquid from said main device;
a source of pressurized air, consisting of a compressor stage of the engine of said helicopter; and
spray nozzles fed, on the one hand, with pressurized lubricating and cooling liquid from said reservoir and, on the other hand, with pressurized gas from said compressor stage, said spray nozzles being able to spray a mist of lubricating and cooling liquid onto certain components of said transmission gearboxes so as temporarily to supplement the defective main device.
In that known system, the lubricating and cooling liquid is contained in said reservoir at the relatively high pressure (at least 10 bar=106 pascal) of the main device and, should the latter fail, is expelled therefrom by the action of the compressed air produced by said compressor stage, the pressure of which has to overcome that of the liquid in the reservoir. The result of this is that the lubricating and cooling liquid sent to said nozzles is at a high pressure, itself equal to at least 10 bar (106 pascal). In consequence, the backup device consumes a significant amount of lubricating and cooling liquid and the reservoir of the backup device needs to be of large capacity, thus causing its mass and bulk to be high.
The object of the present invention is to overcome these disadvantages and to make it possible to produce a lubricating and cooling system which, for a backup device with the same predetermined survival time, requires only a reservoir with a capacity much lower than that of the known system described hereinabove.
To this end, according to the invention, the system for lubricating and cooling a mechanical assembly such as a helicopter transmission gearbox, said system comprising:
a main lubricating and cooling device circulating a stream of lubricating and cooling liquid through said mechanical assembly at a relatively high pressure; and
an auxiliary backup device set in operation automatically or manually when said main device becomes defective, said backup device comprising:
at least one reservoir of lubricating and cooling liquid;
at least one source of pressurized gas;
at least one spray nozzle which is fed, on the one hand, with pressurized lubricating and cooling liquid from said reservoir and, on the other hand, with pressurized gas from said source, which is able to spray onto a mist of lubricating and cooling liquid said mechanical assembly so as temporarily to supplement the defective main device,
is notable in that:
said reservoir of the auxiliary backup device is independent of said main device; and
said spray nozzle is fed with lubricating and cooling liquid from said reservoir at a pressure at most equal to 2 bar (2.105 pascal).
What happens, according to the invention, because the reservoir of the auxiliary backup device is independent of the main device, that is to say because it has no hydraulic communication therewith, is that the lubricating and cooling liquid sent to the spray nozzles can be at a pressure far lower than that of the liquid of the main device, at most equal to 2 bar (2.105 pascal) and, preferably, roughly equal to 1 bar (105 pascal). The result of this is that, for the same predetermined survival time, the consumption of the backup device according to the present invention is far lower than that of the known system, which means that the amount of lubricating and cooling liquid needed for backup purposes is also far lower. As a result, the capacity, the mass and the bulk of the reservoir are considerably reduced.
Such a low pressure of the lubricating and cooling liquid used for backup purposes may be obtained in various ways. For example:
the lubricating and cooling liquid contained in said reservoir is pressurized by said source of pressurized gas feeding said spray nozzle, for example a compressor stage of the helicopter engine, and a pressure and flow limiter is arranged between said reservoir and said spray nozzle; or alternatively
the lubricating and cooling liquid contained in said reservoir is pressurized by an auxiliary gas source, for example a pressurized gas cylinder, independent of said pressurized gas source feeding said spray nozzle, and a pressure and flow limiter is arranged between said reservoir and said spray nozzle; or alternatively still
the lubricating and cooling liquid contained in said reservoir is pressurized by a pump the outlet of which is connected to said reservoir by a bypass comprising a calibrated valve that opens as soon as the outlet pressure of said pump becomes higher than said feed pressure of the spray nozzle.
It is also noted that, because the reservoir of the auxiliary backup device is independent of the main device, it is easy and advantageous for said reservoir to be arranged some distance away from said mechanical assembly so that said lubricating and cooling liquid it contains does not experience the effect of the heat given off by said mechanical assembly.
Thermal degradation of the liquid contained in said reservoir is thus avoided.
When the system according to the present invention comprises a number of spray nozzles, these may be fed with pressurized gas and with lubricating and cooling liquid through a splitter.
In addition, these nozzles may be split into groups and all the nozzles in one group may be fed with pressurized gas and with lubricating and cooling liquid by a common feed device, all said common feed devices themselves being fed with pressurized gas and with lubricating and cooling liquid by said splitter.